The design instructions and design regulations concerning the safety circuits of an elevator are changing, especially as a result of developments in microprocessor technology and software technology. As a consequence of new design instructions and design regulations, the safety circuits of an elevator can be designed based on microprocessor control and on data bus architecture. A number of sensors measuring the operation of an elevator, for example, can be connected to the same data bus, and the operation of the elevator can be monitored with a separate electronic supervision unit connected to the data bus.
The purpose of the safety circuit of an elevator is to ensure the safe operation of the elevator system in all operating situations. In this connection also a high, generally at least SIL 2— or SIL 3—safety integrity level, is required of microprocessor-controlled supervision units, data buses, sensors, et cetera. For achieving an adequate safety level, devices and data buses must often be duplicated; duplication, on the other hand, increases the amount of components, circuits and wiring needed.
As circuits and wiring increase, the risk of a connection error or of a wiring error, or of connection of the wrong voltage to the signal conductors of a data bus, et cetera, also increases. The operating voltage for devices to be connected to a data transfer bus is also often carried in the same cable as the signal conductors. A breakage of the insulation of a conductor might cause a short-circuit or the connection of operating voltage to a signal conductor.
Therefore the safety circuit of an elevator must designed to be fail-safe such that the elevator is always removed from use in a fault situation of the safety circuit. Consequently it would be necessary to ensure that fault situations are identified and located as soon as possible in an early stage, at best already in connection with the installation of the elevator, in which case any fault situation will cause as little harm as possible to operation of the elevator.